1. Field of the Invention
This invention relates to new polyimides formed from the reaction of aromatic dianhydrides with novel diamines containing carbonyl and ether connecting groups between the aromatic rings whereby processable, high strength, solvent, chemical and impact-resistant polyimides are obtained.
2. Description of the Related Art
Polyimides are condensation polymers commonly synthesized by the reaction of aromatic dianhydrides with aromatic diamines. The intermediate polyamide acid is either thermally or chemically cyclodehydrated to form the polyimide which has a repeat unit of the general type ##STR1## Ar is a tetravalent aromatic radical which can be as simple as 1,2,4,5-tetrasubstituted benzene. Ar may be a 4-bis-(o-diphenylene) having the general structure ##STR2## where X=nil, O, S, SO.sub.2, C.dbd.O, and Si(CH.sub.3).sub.2, or Ar may be any other appropriate tetravalent radical. Ar' is a divalent aromatic radical which may be 1,3-phenylene, 1,4-phenylene, 4,4'-biphenylene, 4,4'-oxydiphenylene, 4,4'-thiodiphenylene, 4,4'-carbonyldiphenylene, 4,4'-methanediphenylene, or any other appropriate divalent radical.
Synthesis and characterization of polyimides has been extensively reported in literature. The preparation of aromatic polyimides by reaction of an aromatic dianhydride with an aromatic diamine, followed by thermal cyclization was first reported in 1963 [G. M. Bower and L. W. Frost, J. of Polym. Sci. A1, 3135 (1963)]. Several reviews on polymers have been published [C. E. Sroog, "Polyimides" in Encyclopedia of Polymer Science and Technology (H. F. Mark, N. G. Gaylord, and N. M. Bikales, Ed.), Interscience Publishers, New York, 1969, Vol. 11, pp. 247-272; N. A. Adrova, M. I. Bessonov, L. A. Laius, and A. P. Rudakov, Polyimides, Technomic Publishing Co., Inc., Stamford, Conn., 1970; M. I. Bessnov, M. M. Koton, V. V. Kudryavtsev, and L. A. Laius, Polyimides: Thermally Stable Polymers, Consultants Bureau, New York, 1987.]
Wholly aromatic polyimides are known for their exceptional thermooxidative and chemical resistance. Several polyimides such as Kapton.RTM. (DuPont), PI-2080 (formerly Upjohn, now Dow), XU-218 (Ciba-Geigy), ULTEM.RTM. (General Electric) and LARC-TPI (Mitsui Toatsu) are commercially available and used as films, moldings, adhesives, and composite matrices. As a class of materials, aromatic linear polyimides are generally considered to be amorphous. However, there are numerous examples of polyimides which display crystallinity [Adrova, op. cit., pp. 136-144; T. L. St. Clair and A. K. St. Clair, Journal of Polymer Science, Polymer Chemistry Edition, 15, 1529 (1977)]. Kapton.RTM., polyimide film has been shown to exhibit molecular aggregation or superstructure [S. Isoda, H. Shioda, M. Kochi and H. Kambe, Journal of Polymer Science, Polymer Physics Edition, 19, 1293 (1981); S. Isoda, M. Kochi, and H. Kambe, Ibid., 20, 837 (1982)]. Kapton.RTM. and other reported semi-crystalline polyimides exhibit exceptional thermal stability and resistance to solvents while under stress, but cannot be easily thermoformed into useful molded objects or composites.
The introduction of crystallinity into a polymer has long been recognized as an effective means of improving the solvent resistance and increasing the modulus. In addition, if the proper degree and type of crystallinity is attained, the material can also display extremely high toughness. A notable example is polyetheretherketone (PEEK.RTM. of Imperial Chemical Industries) which exhibits a very high fracture toughness (G.sub.Ic, critical strain energy release rate), and is highly solvent resistant. PEEK.RTM. can also be thermally processed into moldings and composites. The carbonyl and ether connecting groups between the aromatic rings in PEEK.RTM. tend to be so stereochemically similar that the tendency toward crystalline order is greatly enhanced [T. E. Atwood, P. C. Dawson, J. L. Freeman, L. R. J. Hoy, J. B. Rose and P. A. Staniland, Polymer, 22, 2096 (1981)].
It would be advantageous if the benefits of PEEK.RTM. could be obtained from polyimides. Polyimides are more easily prepared and isolated than PEEK.RTM.. Additionally, special technology is required for impregnating PEEK.RTM. into fibers due to the insolubility of PEEK.RTM. in common solvents. The polyamide acid precursor to polyimides is usually soluble and is applied as a solution to fibers or fabric. In this manner good impregnation or "wetting" of the fibers is obtained before thermal cyclodehydration to form an insoluble polyimide. An alternate method involving melt impregnation with the polyimide such as in powder form is also envisioned.
The ratio of carbonyl to ether linkages is critical toward achieving the goal of thermally processable semi-crystalline polyimides. The carbonyl and ether linkages could be incorporated into either the Ar (dianhydride) or the Ar' (diamine) portion of the polyimide repeat unit. However, experience has shown that the diamine portion is easier to modify, resulting in fewer steps than required for the synthesis of new dianhydrides. Novel diamines containing varying ratios of carbonyl to ether groups can be readily synthesized from commercially available materials.
U.S. Pat. No. 4,820,791, Hergenrother, et al, discloses a similar method of making polyimides using aromatic dianhydrides and aromatic diamines with carbonyl and ether groups between the aromatic rings. However, the present invention produces novel polyimides and novel diamines. These new polyimides are not obvious because the improvements over the previous polyimide preparation could not be predicted by the placement of --(CH.sub.2).sub.z -- or --(CF.sub.2).sub.z -- in the center of the diamines or the addition of isopropyl groups to the diamines. The use of --(CH.sub.2).sub.z -- gave a much shorter annealing time, and the use of the --(CF.sub.2).sub.z -- and the isopropyl groups improved the solubility of the materials. Similarly it was not apparent that replacing the carbonyl groups in the diamines with phosphine oxide would be an improvement over the old polyimide by producing a more flame resistant polyimide.